1. Technical Field
The present invention relates to gated systems, and more particularly, to such systems that employ an array of reflective elements for implementing the gating.
2. Discussion of Related Art
Active gated systems are known in the art for achieving an enhanced image of a scene in high scattering or absorption media. Gated systems are used when there is a clear advantage for a reflective image rather than a thermal (emitted) image. Since the human eye is used to perceiving a reflected image and the human brain is accustomed to process reflected images, it is easier to interpret reflected images.
Thermal imagers are associated to emitted image formed by the collection of the photons emitted from the observed target. There are certain features in an image that one can observe only by using the reflected image and equally there are such that can be achieved only by using the emitted image.
Active imaging benefits from a unique technological feature that enables the synchronized switching between the light source and the camera. This mode of operation is referred to as synchronized gated imaging (SGI) or burst illumination (BIL). The active imaging systems mode eliminates the reflected backscatter of near range reflectors. A reflector may be an aerosol particle or any feature located within the field of view. The SGI mode of operation enables adjustments to the illumination level at each range resulting in an effective uniform illumination regardless of the range. The depth of field is a controllable feature of an active system, controlling the opening and closing of the camera and light source in a synchronized manner along the time line.
If the transparent atmosphere medium is clear there is no need for gating. When observing a target with known range with no obstacles along the line of sight there will be no reflections of close objects. When there are reflections from close objects, the gating technique eliminates the backscatter target contrast degradation.
FIG. 1 is a schematic block diagram illustrating the reflection due to an obstruction media according to the existing art. An exemplary gating imaging system 10 operates as follows: pulse of light (can be laser) 13 from illuminator 12 is radiated to the atmosphere. Some of the pulses backscatter from a disturbing medium 16. In order to eliminate the impact of the backscattering, the camera shutter 14 is closed when the backscattering radiance reaches it and the camera shutter opens when the pulse 14 returns after reflection from target 17.
There are several known methods in the art to design a gated imaging system. One method is based a single pulse per frame—in one camera frame time (normally for standard video about 30-40 msec) only one pulse of laser is radiated to the target. The camera is synchronized for the return of the pulse. Usually the laser has high energy per pulse and very narrow pulse width (˜20-100 nsec). The implementation of this method compels the use of a detector so that its internal shutter has a response time in the order of micro seconds and possibly less.
Another method is based on multiple pulses per frame—in one camera frame time multiple pulses of light (normally laser) are radiated to the target with time delay between one another. The camera is synchronized for the return of each pulse. The time delay between the gate “ON” duration of the camera and the radiation of the light source is depended on the distances to the observed scene. The duration of the “ON” time is also depended on the distance. The light source can be operated in high repetition rates (even up to mega hertz) with high average power and changeable pulse width (typically 100 nsec to 50 microsec for observation systems or even femto-second for very small depth of filed imaging). The implementation of this method compels the use of specific and unique types of detectors. This is because the internal shutter needs to be opened and closed in the same repetition rate of the light source (even up to mega hertz). The common sensors that are being used in a multiple gating system are ICMOS/ICCD/EBAPS (which has this capability). In these sensors the image intensifier (II) behaves as the shutter in front of the camera (The II has very fast shuttering capabilities). The spectral sensitivity is limited to the image intensifier sensitivity. This method is illustrated in FIG. 2A showing the timing scheme of the gating and the light source signal over time.
The laser and camera are synchronized in time. The depth of field and minimum range can be achieved by changing the synchronization and time scheme.
As illustrated in FIG. 2B and FIG. 2C, there is a possibility to change the depth of field and minimum range from frame to frame by playing with the timing. In this way a 3D video is achieved. The 3D video can be used for better understanding of the scene and the distance of detected objects. Moreover this method will produce better imaging performance—the illumination will be uniform over the entire depth of field. For every depth slice the illumination timing and power is optimized. All the slices can be combined to generate one image.